The invention concerns a device for processing workpieces using ultrasound, comprising an ultrasound generator, an ultrasound sonotrode, and an resonant system comprising an anvil, wherein a workpiece is processed between the anvil and the ultrasound sonotrode, and with a regulation means for the ultrasound generator, the regulation means comprising a regulation member connected upstream of the ultrasound generator, which receives a feedback signal from the resonant system, generates a regulation variable, and supplies it to the ultrasound generator.
The invention concerns a regulation device for an ultrasound generator for processing workpieces, in particular, welding or cutting them. An ultrasound generator in an resonant system generates oscillations, using an electroacoustic converter, within its resonance frequency, i.e. usually between 18 and 60 kHz. These oscillations operate an ultrasound sonotrode either directly or via a booster or converter. The ultrasound sonotrode, together with the anvil, represent the tool for processing the workpiece.
Conventional ultrasound generator regulation means measure certain system parameters and system signals which they obtain as information from the electrical and mechanical resonant system. These signals and parameters regulate the ultrasound generator. Changes within the resonant system are thereby detected and the resonant system is reset at the original value through suitable algorithms. Internal system parameters and signals are e.g. the resonant frequency, the current, the reactive current, the voltage, the phase position etc.
The main aim of all resonant systems is to keep the oscillation amplitude at a constant value with maximum efficiency. Since the resonance frequency of the overall resonant system, which consists of a mechanical and an electrical resonant circuit, constantly changes during operation, the regulation process must be permanently repeated and be as fast as possible. The changes or disturbances in the resonant system are due to load changes (e.g. force fluctuations), changes of the geometry due to heating, fluctuations in the energy supply etc. The regulation improves with accelerated repetition rate of the regulation process and better adjustment of the regulation algorithm. Oscillation of the regulation circuit must be prevented in any case. This means, however, that the regulation parameters cannot be set at excessively high values.
Prior art discloses analog and digital systems (DE-A-40 25 637, DE-A-44 00 210, DE-A-42 30 491, DE-A-42 08 669, EP-A-0 173 761 and U.S. Pat. No. 4,808,948). Irrespective thereof, the parameters provided in the resonant system are the basis for regulating the ultrasound generators. These conventional resonant systems are disadvantageous in that very fast load changes often cause considerable amplitude fluctuations. These fluctuations can, in case of excess, cause the limit values of the mechanical components to be exceeded, thereby damaging them or resulting in insufficient welding.
It has turned out that the regulation does not react sufficiently rapidly. This is mainly due to the system-related parameters and signals. These change with load changes, but reach the regulation member of the ultrasound generator with a certain delay. This delay is mainly predetermined by the speed of sound and the number of mechanical oscillation components.
This means, however, that the regulation member reacts to an event under extreme conditions only when this event is over. Keeping the oscillation amplitude constant on the basis of the system parameters and signals is very difficult. Fast load changes of less than 5 ms therefore produce considerable fluctuations in the oscillation amplitude.
It is therefore the underlying purpose of the invention to further develop a device of the above-mentioned type in such a fashion that it reduces the amplitude fluctuations during fast load changes.